297 research outputs found
Order Parameter at the Boundary of a Trapped Bose Gas
Through a suitable expansion of the Gross-Pitaevskii equation near the
classical turning point, we obtain an explicit solution for the order parameter
at the boundary of a trapped Bose gas interacting with repulsive forces. The
kinetic energy of the system, in terms of the classical radius and of the
harmonic oscillator length , follows the law , approaching, for large , the
results obtained by solving numerically the Gross-Pitaevskii equation. The
occurrence of a Josephson-type current in the presence of a double trap
potential is finally discussed.Comment: 11 pages, REVTEX, 4 figures (uuencoded-gzipped-tar file) also
available at http://anubis.science.unitn.it/~dalfovo/papers/papers.htm
Parity Invariance and Effective Light-Front Hamiltonians
In the light-front form of field theory, boost invariance is a manifest
symmetry. On the downside, parity and rotational invariance are not manifest,
leaving the possibility that approximations or incorrect renormalization might
lead to violations of these symmetries for physical observables. In this paper,
it is discussed how one can turn this deficiency into an advantage and utilize
parity violations (or the absence thereof) in practice for constraining
effective light-front Hamiltonians. More precisely, we will identify
observables that are both sensitive to parity violations and easily calculable
numerically in a non-perturbative framework and we will use these observables
to constrain the finite part of non-covariant counter-terms in effective
light-front Hamiltonians.Comment: REVTEX, 9 page
Application of Pauli-Villars regularization and discretized light-cone quantization to a single-fermion truncation of Yukawa theory
We apply Pauli-Villars regularization and discretized light-cone quantization
to the nonperturbative solution of (3+1)-dimensional Yukawa theory in a
single-fermion truncation. Three heavy scalars, including two with negative
norm, are used to regulate the theory. The matrix eigenvalue problem is solved
for the lowest-mass state with use of a new, indefinite-metric Lanczos
algorithm. Various observables are extracted from the wave functions, including
average multiplicities and average momenta of constituents, structure
functions, and a form factor slope.Comment: 21 pages, 7 figures, RevTeX; published version: more extensive data
in the tables of v
Conserving and Gapless Approximations for an Inhomogeneous Bose Gas at Finite Temperatures
We derive and discuss the equations of motion for the condensate and its
fluctuations for a dilute, weakly interacting Bose gas in an external potential
within the self--consistent Hartree--Fock--Bogoliubov (HFB) approximation.
Account is taken of the depletion of the condensate and the anomalous Bose
correlations, which are important at finite temperatures. We give a critical
analysis of the self-consistent HFB approximation in terms of the
Hohenberg--Martin classification of approximations (conserving vs gapless) and
point out that the Popov approximation to the full HFB gives a gapless
single-particle spectrum at all temperatures. The Beliaev second-order
approximation is discussed as the spectrum generated by functional
differentiation of the HFB single--particle Green's function. We emphasize that
the problem of determining the excitation spectrum of a Bose-condensed gas
(homogeneous or inhomogeneous) is difficult because of the need to satisfy
several different constraints.Comment: plain tex, 19 page
Exploring Mars at the nanoscale: applications of transmission electron microscopy and atom probe tomography in planetary exploration
The upcoming Mars Sample Return (MSR) mission aims to deliver small quantities of Martian rocks to the Earth. Investigating these precious samples requires the development and application of techniques that can extract the greatest amount of high quality data from the minimum sample volume, thereby maximising science return from MSR. Atom probe tomography (APT) and transmission electron microscopy (TEM) are two complementary techniques that can obtain nanoscale structural, geochemical and, in the case of atom probe, isotopic information from small sample volumes. Here we describe how both techniques operate, as well as review recent developments in sample preparation protocols. We also outline how APT has been successfully applied to extraterrestrial materials in the recent past. Finally, we describe how we have studied Martian meteorites using TEM and APT in close coordination in order to characterise the products of water/rock interactions in t h e cru st of Ma r s – a k ey sc ie n ce goal of MSR. Our results provide new insights into the Martian hydrosphere and the mechanisms of anhydrous-hydrous mineral replacement. In light of the unique results provided by these tools, APT and TEM should form a crucial part at the culmination of a correlative analytical pipeline for MSR mission materials
A Study of Heavy-Light Mesons on the Transverse Lattice
We present results from a study of meson spectra and structure in the limit
where one quark is infinitely heavy. The calculations, based on the framework
of light-front QCD formulated on a transverse lattice, are the first
non-perturbative studies of B-mesons in light-front QCD. We calculate the
Isgur-Wise form factor, light-cone distribution amplitude, the heavy-quark
parton distribution function and the leptonic decay constant of B-mesons.Comment: 5 pages, 3 figures, Revtex, corrected typos, added references,
included moment
Condensate fraction and critical temperature of a trapped interacting Bose gas
By using a mean field approach, based on the Popov approximation, we
calculate the temperature dependence of the condensate fraction of an
interacting Bose gas confined in an anisotropic harmonic trap. For systems
interacting with repulsive forces we find a significant decrease of the
condensate fraction and of the critical temperature with respect to the
predictions of the non-interacting model. These effects go in the opposite
direction compared to the case of a homogeneous gas. An analytic result for the
shift of the critical temperature holding to first order in the scattering
length is also derived.Comment: 8 pages, REVTEX, 2 figures, also available at
http://anubis.science.unitn.it/~oss/bec/BEC.htm
Penguins leaving the pole: bound-state effects in B decaying to K* + photon
Applying perturbative QCD methods recently seen to give a good description of
the two body hadronic decays of the B meson, we address the question of
bound-state effects on the decay B into K* + gamma. Consistent with most
analyses, we demonstrate that gluonic penguins, with photonic bremsstrahlung
off a quark, change the decay rate by only a few percent. However, explicit
off-shell b-quark effects normally discarded are found to be large in
amplitude, although in the standard model accidents of phase minimize the
effect on the rate. Using an asymptotic distribution amplitude for the K* and
just the standard model, we can obtain a branching ratio of a few times
10^{-5}, consistent with the observed rate.Comment: 12 pages. U. of MD PP \#94-129; DOE/ER/40762-033; WM-94-104. LaTeX,
One figure, available by fax or pos
Gravitons in One-Loop Quantum Cosmology: Correspondence Between Covariant and Non-Covariant Formalisms
The discrepancy between the results of covariant and non-covariant one-loop
calculations for higher-spin fields in quantum cosmology is analyzed. A
detailed mode-by-mode study of perturbative quantum gravity about a flat
Euclidean background bounded by two concentric 3-spheres, including
non-physical degrees of freedom and ghost modes, leads to one-loop amplitudes
in agreement with the covariant Schwinger-DeWitt method. This calculation
provides the generalization of a previous analysis of fermionic fields and
electromagnetic fields at one-loop about flat Euclidean backgrounds admitting a
well-defined 3+1 decomposition.Comment: 29 pages, latex, recently appearing in Physical Review D, volume 50,
pages 6329-6337, November 1994. The authors apologize for the delay in
circulating the paper, due to technical problems now fixe
The BRST quantization and the no-ghost theorem for AdS_3
In our previous papers, we prove the no-ghost theorem without light-cone
directions (hep-th/0005002, hep-th/0303051). We point out that our results are
valid for more general backgrounds. In particular, we prove the no-ghost
theorem for AdS_3 in the context of the BRST quantization (with the standard
restriction on the spin). We compare our BRST proof with the OCQ proof and
establish the BRST-OCQ equivalence for AdS_3. The key in both approaches lies
in the certain structure of the matter Hilbert space as a product of two Verma
modules. We also present the no-ghost theorem in the most general form.Comment: 22 pages, JHEP and AMS-LaTeX; v2 & 3: minor improvement
- …